A novel palladium-catalyzed asymmetric cyclocarbonylation of allylic alcohols to γ-butyrolactones

A catalyst system based on [Pd2(dba)3]·CHCl3/(-)-BPPM has been found to effect asymmetric cyclocarbonylation of certain prochiral allylic alcohols to produce good yields of optically enriched γ-butyrolactones. The reaction was performed under an atmosphere of H2(400 psi) and CO (400 psi) at 100 C in methylene chloride for 48 hours. Asymmetric cyclocarbonylation of allylic alcohols with aliphatic substituents proceeded with moderate enantioselectiveness (ee = 25-43%). However, enantiomeric excesses of up to 83% were obtained for substrates containing aromatic substituents, in which case the ee was found to be more sensitive to steric, rather than to electronic factors. Recrystallization of the lactones containing an aromatic group from a mixture of CH2Cl2/Et2O/hexanes (0.5/1.0/8.5), by slow evaporation of the solvent or at low temperature, improved the enantiopurities to >98% ee on a reproducible basis. The assymetric center of the aromatic lactones was assigned the (S)-configuration based on the X-ray crystal structure analysis of enantiopure (S)-(+)-3,3-dimethyl-2-(2'-methylphenyl)-γ-butyrolactone (2k). A hydridopalladium intermediate is believed to play a key role in this reaction. Enantioselectivity is thought to be brought about by the preferential formation of 6b. The carbon skeleton of 6b fits into the chiral scaffold of (-)-BPPM.